The invention is related to field of wireless systems, and in particular to a wireless power receiver that can independently make a measurement of instantaneous available power.
In a resonant wireless power (RWP) system, multiple receiver devices can simultaneously receive power from a single source through near-field magnetic coupling. The coupling factor or mutual inductance of each receiver coil to the source coil may vary according to the relative position and orientation of the coils. Even if the receiver coils do not couple directly to each other, each can affect the power transfer of the other through interactions with the source coil. The source is necessarily limited in the amount of power it can deliver to receivers. Thus the available power to any one receiver device at a given time is a function not only of its orientation with respect to the source coil, but also the state of other receiver coils in the system.
In order for the wireless power system to provide a good user experience, an assessment must be made of this available power. To understand why this is the case, consider a hypothetical wireless power system in which available power is not known. Suppose that a device is introduced into the wireless power field for the purpose of charging its battery. A voltage may appear at the output of the wireless power converter, indicating to the receiver device that power is available to charge the battery. However, suppose that conditions are such that only 0.5 W can be drawn from the wireless power source. If the device uses 1 W to charge its battery, initiating charging will cause the power system to collapse. The voltage at the output of the wireless power converter will drop out, causing charging to terminate. It may then recover, leading to limit-cycling behavior. Furthermore, the overloading on the wireless power source may affect the power transfer to other devices in the system.